Virtually all of the fixtures used to light stage shows, concerts, films, and television productions produce light beams of directional character, beams whose azimuth and elevation relative to the structure supporting the fixture must be adjusted in order to illuminate the desired subject. In almost every case, the elevation adjustment capability is provided by suspending the fixture housing in the opening of a fork-like yoke or frame. The required azimuth adjustment capability is provided by rotatably mounting the yoke to the supporting structure. The fixture housing is manually rotated in each axis and locked in place by friction at the pivots, for example, as disclosed in U.S. Pat. No. 1,977,883.
This method of adjustment is simple and inexpensive, but it has the disadvantage that the maximum azimuth adjustment on either side of a plane perpendicular to the long axis of a common support, for any given fixture length greater than the fixture width, is inversely related to the spacing between the fixture and an adjacent fixture or obstruction. This requires that the spacing between adjacent elongated fixtures on a common support be increased in order to provide adequate clearance for azimuth adjustment, with the undesirable effect of thus decreasing the number of fixtures which can be accomodated on a given support.
Those fixtures which allow remote adjustment of azimuth and elevation generally employ this method with the addition of motors at the pivots, for example, as disclosed in U.S. Pat. Nos. 1,680,685 and 1,747,279.
This method of remote azimuth and elevation adjustment has several important disadvantages. One is that the entire mass of both the fixture and its yoke must be suspended from the azimuth pivot, and a motor and drive system must be provided having sufficient torque to accelerate this mass to motion and then smoothly decelerate it to a stop at the required position with the contradictory objects of maximum speed and accuracy. The azimuth pivot must further maintain a high degree of stiffness so as not to introduce undesirable motion or error into positioning while minimizing friction, requiring the use of expensive bearings. Further, the very length of the fixture housing increases the problem of inertia presented by its moving mass. Also, changes in the design of the fixture which increase its length, or weight, or change its center-of-gravity can have such impact on the requirements for the azimuth actuator that major changes in the actuator and/or drive may be required. One such change is the addition of a motorized color changer mounted to the front of the fixture. Finally, most productions demand far more azimuth adjustment of a fixture than elevation adjustment, placing the highest workload on the weakest link in the system.
Alternatively, some fixtures have been built with the use of a mirror rotatable in two axes as the method of adjusting azimuth and elevation, a remotely adjustable version disclosed in U.S. Pat. No. 2,054,224. While the motorization of such a fixture is far simpler because of the minimal mass of the mirror, this approach sets severe and generally unacceptable limits on adjustability, for while the azimuth adjustment of such a system is unrestricted, elevation adjustment is limited to within a narrow range on either side of a plane perpendicular to the elongated axis of the fixture housing. Adjustment towards the fixture results in obstruction of the beam by the fixture housing itself, while adjustment away from it results in clipping of the beam as it elongates beyond the edges of the mirror.
It is the object of the present invention to disclose an improved apparatus for adjusting fixture azimuth and elevation, manually or remotely, having none of the disadvantages of prior art methods.